1. Field of the Invention
The present invention relates to a process for manufacturing a siamese-type cylinder block and more particularly, to such a process comprising a blank making step of providing a cylinder block blank in which a sleeve made of a cast iron is incorporated cast in each cylinder barrel of a siamese-type cylinder barrel made of an aluminum alloy and consisting of a plurality of cylinder barrels connected in series, and a mechanical working or machining step of forming the inner peripheral surface of each sleeve of the resulting cylinder block blank into a true circle.
2. Description of the Prior Art
Such conventional blank making steps include placing sleeves in a siamese-type cylinder barrel molding cavity in a mold and then, pouring a molten metal of aluminum alloy under pressure into the cavity for casting. Thereby, a casting strain is produced in the cylinder barrels in the blank due to the casting pressure and the action of rapid solidification of the aluminum alloy. With a sleeve having a smaller thickness and a lower rigidity, such a casting strain influences the sleeve to produce a strain therein. To avoid this, the thickness of the sleeve may be increased, but with a too large thickness, the amount of sleeve to be cut is increased in subsequent working into a true circle, which is uneconomical and causes an increase in working time. Even if the thickeness of the sleeve is increased, there are the following problems which arise with a cylinder block resulting from the immediately working of the inner peripheral surface of the sleeve into a true circle after the casting of the blank. In the operation of an engine assembled using such cylinder block, the casting strain in the cylinder barrel influences the sleeve when the cylinder barrel heated during the operation has been returned to an ambient temperature after the stoppage of operation of the engine, thereby causing the amount of permanent deformation of the inner diameter at the sleeve to increase. Thus, a clearance is produced between a piston ring and the sleeve resulting in an increased amount of blow-by gas and a useless consumption of oil.
When the sleeve has increased thickness at an ambient temperature, the heat of molten metal is absorbed by the sleeve so that the molten metal close to the sleeve is solidified earier than the molten metal close to a breakable core for forming a water jacket. Consequently, the metal structure in the cylinder barrel is different from that at the portion close to the core. In this case, both the metal structures around the sleeves vary in thickness in the radial direction of the sleeve, and because the region between the adjacent sleeves is not occupied by the core, the metal structure between the adjacent sleeves is different from both the above metal structures. In addition to the problem in metal structure, because the shrinkage of the sleeve heated by the molten metal dose not follow the solidification shrinkage of the molten metal, the casting stress remaining the sleeve is not uniform around the circumference of the sleeve.
The absorption of the heat of the molten metal by the sleeve causes the early solidification of the molten metal to degrade the close adhesion between the sleeve and the molten metal, thereby producing a very small clearance between the sleeve and the cylinder barrel resulting in a poor release of heat of from the sleeve.
Thus, if the casting stress remaining in the sleeve is not uniform around the circumference from the sleeve the release of heat of the sleeve is poor, and in the operation of an engine assembled using a cylinder block obtained through the working of the inner peripheral surface of such sleeve into a true circle, the amount of sleeve thermally expanded is ununiform around the circumference of the sleeve, causing a clearance to be produced between a piston ring and the sleeve, resulting in the same problems as described above.
In providing a blank as described above and including a water jacket to which the entire periphery of a siamese-type cylinder barrel faces, operations which have been adopted include placing sleeves and a water-jacket shaping breakable core surrounding the sleeves into a siamese-type cylinder barrel molding cavity in a mold and then, pouring a molten metal of aluminum alloy into the cavity to cast a blank, removing unnecessary portions such as gates and runners from the blank and then, breaking the breakable core to remove about half thereof by applying vibration to the blank, and heating the blank for a period of about 4 hours at a temperature of 350.degree. C. or more to burn a binder contained in the core and enhance the breakability of the remainder of the core. In the above heating step, the heating causes the hardness of the aluminum alloy portion in the blank to be considerably reduced and make it impossible for a cylinder head-bound surface, a crank journal bearing holder, an oil pan-bound surface of a crankcase or the like to retain a satisfactory hardness. Therefore, the heating step has been followed by an operation comprising subjecting the blank to a T6 treatment, namely to a thermal treatment of heating the blank for a period of about 2 hours at a temperature of about 500.degree. C. and then cooling it with water to provide the recovery of the hardness, a step of breaking the remainder of the core to remove it from the blank by applying vibration to the blank, subjecting the blank to cleaning fettling and checking the resulting blank.
However, the above conventional process is accompanied by a problem that even if the T6 treatment enables the hardness of the aluminum alloy portion in the blank to be improved, a non-uniform stress remains in the sleeve at the cooling step in the above treatment and thus, a high performance cylinder block can not be obtained.
The conventional process also has the disadvantage of uneconomically increased amount of energy consumed due to two heating steps included therein.